BENTHIC HETEROTROPHIC NANOFLAGELLATES IN NORTH-SEA FIELD MESOCOSM BOTTOMS AND THEIR RESPONSE TO ALGAL SEDIMENTATION

We studied densities of heterotrophic benthic nanoflagellates (< 20-mu-m) in relation to densities and productivity of bacterial communities in North Sea bottoms. Experimental mesocosm bottoms, collected at a sandy (depth 28 m) and a silty (depth 38 m) locality, received one substantial input of organic matter (23 g C dead phytoplankton m-2) simulating spring phytoplankton sedimentation. Flagellate densities and bacterial productivity/numbers were measured in the surface layers of the mesocosm sediments and in fresh sediment cores from the field. A clear pattern emerged for the top layer in the field sandy sediments: flagellate densities were low in April (50 x 10(3) cm-3), high during June and August (300 x 10(3) cm-3), and low again in November. In the mesocosm sandy sediments numbers had increased from low to high after the simulated algal bloom in June, but they were again low in August and through the remainder of the season. There were no variations in the flagellate densities deeper in the sediment. In the silty sediments flagellate densities were low in the field and in the mesocosm. High densities of these benthic flagellates were invariably parallelled by high benthic bacterial production, but not vice versa. Flagellate bacterivory could have considerable impact on bacterial production in the sandy North Sea bottom but not in the muddy sediment.